1. Field of the Invention
The present invention relates to a contact lens, and relates in particular to a multifocal contact lens having a plurality of diopter power regions.
2. Description of the Related Art
It is commonly understood that in order to correct presbyopia, for example, multifocal contact lenses such as bifocal lenses will be prescribed. These lenses include a distance region provided as a first diopter power region and a near region provided as a second diopter power region having a more positive additional diopter power than that of the distance region.
For example, EP-A2-0201231 discloses such multifocal contact lenses with a configuration where a distance region and a near region are formed in a concentric pattern about a lens geometric center. On the other hand, it is known that the center of the pupil is slightly biased from the center of the cornea toward the nose. Consequently, with the configuration as taught in EP-A2-0201231, the two regions do not have an intended positional relationship with respect to the pupil during actual wear, because the distance region and the near region are formed in a concentric pattern about the lens geometric center on the assumption that the lens geometric center is to be aligned with the center of the pupil. As a result, it is not possible to afford an effective correction neither for distance vision nor for near vision.
Accordingly, in Japanese unexamined Patent Publication No. JP-A-6-289329, the Assignee proposed multifocal contact lenses wherein a near region is decentered from a lens geometric center toward the nose. With this arrangement, both the distance vision and the near vision can be effectively corrected by aligning the visual axis of the eye with the lens optical axis, being advantageously correspondent with the visual axis of the eye shifting toward the nose when looking at something near. However, the multifocal contact lenses disclosed in JP-A-6-289329 may have a near region being decentered not only in the lateral direction but also in the vertical direction of the wearer. Accordingly, it will be necessary to provide the lenses for use in the right eye and in the left eye individually, requiring additional specifications for the left and right eyes. Thus, if the lenses are supplied through made-to-stock production, a lot of stocks need to be prepared, posing a risk of increase in production cost or management cost.
A general problem with the multifocal contact lenses disclosed in EP-A2-0201231 or JP-A-6-289329 is that since the multifocal contact lens includes one optical region with a plurality of diopter power regions, each diopter power region has a narrow effective optical region. Consequently, optical characteristics are not as good as those of monofocal lenses. For example, many of the patients who now wear multifocal lenses for correction of presbyopia have corrected only distance vision with monofocal lenses so far. By using multifocal lenses, they become more satisfied with their near vision, whereas they are discontent with their distance vision compared to that by using monofocal lenses. In particular, during the night, their distance vision suffers from flare that occurs due to defocus of the near region, causing poor visibility. Thus, many of users of the multifocal lenses wear monofocal lenses when they drive or play sports at night, which obliges them to own both multifocal lenses and monofocal lenses for alternative use.
Another risk associated with the multifocal contact lenses is that correction may vary depending on changes in environment such as changes of brightness from day to night, or changes in usage such as changes from operating PC indoors to watching sports outdoors. Namely, when providing prescription for multifocal contact lenses, while the corrective diopter powers will be decided by an objective examination or a subjective examination, the corrective diopter power for near vision in particular is often decided with a great regard for the wearers' vision in the subjective examination. Here, in most cases, the multifocal contact lens will be prescribed indoors. Consequently, the multifocal contact lenses optimized under the environment at the time of prescription pose a risk that the wearers become less satisfied with their vision which changes under the different environment or usage conditions.
Furthermore, since multifocal contact lenses have a plurality of diopter powers to be adjusted, the prescription needs more time and labor than in case of monofocal lenses. Therefore, the multifocal contact lenses tend to be avoided by prescribers. Similarly, for the wearers, it takes time to get used to the multifocal contact lenses because of the unique vision thereof different from that of monofocal lenses.
An alternative method of correcting presbyopia is monovision prescription as disclosed in U.S. Pat. No. 5,296,881, for example. In monovision prescription, a contact lens adjusted for distance vision is adapted to be fitted to one eye while a contact lens adjusted for near vision is adapted to be fitted to the other eye. By so doing, the one eye will be given distance vision while the other eye will be given near vision. With this arrangement, it is possible to see the desired image by selecting from the image obtained through the distance vision and the image obtained through the near vision according to the action of wearers' brain. However, monovision prescription has not become widespread because of requirement of time and labor for prescription as well as difficulty in choice of lenses suitable for environment of usage.